Study On The Soot Formation Characteristics At Low-Temperature Combustion For Diesel Engine

As environmental pollution increases and emissions regulations become more stringent,Low temperature combustion strategies have been extensively studied because of its potential for increasing thermal efficiency and reducing emissions.In this paper,the soot formation and oxidation process of a diesel engine under low temperature combustion conditions are extensively explored by using the CONVERGE code.The effects of EGR rates,fuel injection timing and intake valve close timing on the characteristics of soot generation process were studied.The main findings are as follows:The ignition delay times,laminar flame speeds and intermediate species concentrations predicted by four chemical reaction mechanisms are compared and analyzed by using the CHEMKIN-PRO Code.The mechanism with the best predictions is selected as the diesel fuel surrogate mechanism.After that,the constants in the spray model,combustion model and smoke formation and oxidation model are calibrated with experimental data respectively.The effects of EGR rates on the combustion and emission characteristics of the diesel engines are studied.With the increase of the EGR rate,the mass of soot emission decreases,while the number and density of soot particles increase.However,the average diameter of the soot particles increases first and then decreases.Soot emission is a combination result of the formation and oxidation reactions.When the EGR rate is less than 50.0%,soot oxidation reaction plays a leading role,while when the EGR rate is more than 50.0%,soot formation process dominates.The effect of diesel fuel injection timing on soot emission characteristics of a diesel engine is studied.With the increase of fuel injection advanced angle,soot formation and oxidation rate are both increased.The soot number density and soot average diameter are both decreased.When the injection angle is larger than 30°CA BTDC,the NOx and soot emission can be reduced simultaneously.The soot emission characteristics of diesel engine are studied with different intake valve closure timings.The results show that the effective compression ratio of the diesel engine decreases with the retreating of intake valve closure timing.The mass,density and average diameter of soot particles and the NOx emission are all decreased simultaneously with the retreating of the inlet valve closing timing.When the inlet valve closing angle is less than 100°CA BTDC,the emissions of NOx and soot can meet the requirements of Euro-6 emission regulations.The effects of EGR rate,fuel injection timing and intake valve closing timing on soot generation characteristics of diesel engine are studied.EGR,early fuel injection and intake valve closing timing can reduce NOx emissions simultaneously,its degree of influence is decreasing.Early fuel injection and intake valve closing timing can reduce soot emissions,however,EGR increases the soot emissions,the effect of early fuel injection is less than intake valve closing timing.The combined effects of higher EGR rate,earlier injection timing and later intake valve closing technologies can effectively reduce soot particle number,density and average diameter.